WPF could not get touch position from MouseDown event

Question

I have a WPF project written before touch support was added to .NET (v 4.0), so only the mouse events were handled. I run into this problem when testing the project on a touch screen with fingers.

The problem is, the position (X, Y) is correctly retrieved in the first touch, but the (X, Y) values stay the same in subsequent touches, no matter where I touch, and even if I touch out of the Image, the MouseDown event is fired, which makes it more weird.

It can be reproduced with .NET 3.0/3.5/4.0, tested on Win7/Win8, both are 64 bits. And it seems it is MouseDown event that is misbehaving, MouseUp works fine.

Update:

This is a bug with long history and MS has not yet fixed it (even in 4.5), so you have to update the code if you encounter the same symptom - get the touch position from Touch event, not Mouse event. Luckily this bug is not subtle so it only takes a little while to be located and fixed.

Code to reproduce the issue:

XAML:

<Window x:Class="WpfApplication1.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="MainWindow" Height="350" Width="525">
    <Grid>
        <Image Height="60" Width="80" x:Name="Image" MouseDown="Image_MouseDown" 
                Source="/WpfApplication1;component/Images/Desert.jpg" />
    </Grid> 
</Window>

Code behind:

using System;
using System.Collections.Generic;
using System.Text;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;

namespace WpfApplication1
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }
        private void Image_MouseDown(object sender, MouseButtonEventArgs e)
        {
            Point p = e.GetPosition(Image);
            MessageBox.Show(p.X.ToString() + " " + p.Y.ToString());
        }
    }
}

score 2 · Accepted Answer · answered Feb 17 '14 at 12:19

By default in WPF, if a Touch event is not handled by a control it will be promoted to a Mouse event. Touch events are routed events, so when the Mouse event will be triggered it will go up and down the visual tree (hence your event handler being executed even when you touch outside of the Image).

If the MouseDown event comes from a promoted touch event, you could probably get the correct position using the StylusDevice:

if (e.StylusDevice != null)
    point = e.StylusDevice.GetPosition(sender as Image);

Or as an alternative, you could add a handler for the TouchDown event for the controls where you need the position:

<Image TouchDown="UIElement_OnTouchDown"/>

 private void UIElement_OnTouchDown(object sender, TouchEventArgs e)
 {
      var touchPoint = e.GetTouchPoint(sender as Image);
      // more processing using touchPoint.Position
 }

Yes, handling TouchDown event works for me. But it looks to me more like an obvious bug in framework itself, because legacy code that works well with mouse click is expected to work well with touching. — kennyzx, Feb 17 '14 at 15:14
@kennyzx I was able to reproduce this with a sample new project, so yes it looks like an issue or just the way touch support was added. — Adrian Fâciu, Feb 18 '14 at 08:08

score 0 · Answer 2 · answered Nov 19 '14 at 17:12

0

If you run in .NET 4.5.0, you will not see this issue. It is introduced at 4.5.1. I am trying to see if I can get somebody at MS to pay attention and fix this without you and me hacking up our code.

answered Nov 19 '14 at 17:12

osoblanco

468
2
10

I will check it out in 4.5 – kennyzx Nov 20 '14 at 17:06
Unfortunately, it makes no difference, I have tried re-targeting 4.5 and 4.5.1, no luck. – kennyzx Nov 21 '14 at 02:06
I apologize. I spent too much time messing with this and got confused. The fix of adding the touchdown is required and sufficient for 4.5.0. Meanwhile, in 4.5.1 and beyond I also have to add explicit stylusdown handling as well. – osoblanco Nov 24 '14 at 19:08

WPF could not get touch position from MouseDown event

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